1. Field of the Invention
This invention relate, in general, to rare-gas arc lamps wherein a rare gas, e.g., argon or xenon, is sealed. The rare-gas arc lamp produces a positive column between a pair of electrodes for radiating light.
2. Description of the Prior Art
A xenon glow lamp and a fluorescent lamp are used for a light source of an apparatus, such as, e.g., copying machine, facsimile device, etc. The xenon glow lamp and the fluorescent lamp also are used for a backlighting of a liquid crystal display. As is well known, in a typical fluorescent lamp, a pair of coil filaments are respectively arranged at opposite ends of an elongated bulb, the inner surface of which is coated with a fluorescent material. A fill including mercury and a rare-gas, e.g., argon is sealed in the bulb. In such a lamp, the amount of ultraviolet rays produced is closely related to temperature. This is because the vapor pressure of mercury depends upon the ambient temperature, and the amount of ultraviolet rays is a function of the vapor pressure of mercury within the bulb. Therefore, the luminous efficiency of the lamp extremely decreases when the ambient temperature decreases under 5.degree. C. or increases above 60.degree. C. In an extremely low temperature atmosphere, the starting ability of the lamp greatly decreases, and thus, the starting voltage of the lamp becomes high. Furthermore, since a fluorescent lamp uses the Penning effect of argon gas, argon gas is sealed in the bulb at a prescribed low pressure less than 5 Torr. A Faraday dark space exists in front of an electrode (cathode) because of the low sealing pressure of argon gas. Such a dark space extends approximately 10 mm in the lamp. Since this dark space does not contribute to the radiation, the effective luminescence length of the lamp relatively decreases.
Since the fluorescent lamp includes a pair of hot cathode type electrodes composed of a coil filament, as described above, an electron emissive material, e.g., barium oxide, applied to the coil filament easily evaporates and is adhered onto the inner surface of the bulb when the temperature of the coil filament increases above a prescribed value during operating. Therefore, the inner surface of the bulb becomes black by the accumulation of the electron emissive material.
The xenon glow lamp does not have such disadvantages described above. A conventional xenon glow lamp typically includes a pair of cold cathode type electrodes respectively disposed at opposite ends of a bulb. A rare-gas mainly including xenon gas is sealed in the bulb. A fluorescent layer is formed on the inner surface of the bulb. In such a xenon glow lamp, since the rare-gas is sealed in the bulb at a relatively high pressure greater than 50 Torr, the xenon glow lamp has less temperature dependency compared with the fluorescent lamp described above. However, the starting voltage of the xenon glow lamp is high because of the high sealing pressure. Furthermore, a lamp current is limited to a relatively low value due to the cold cathode type electrode. If the lamp current increases, the cold cathode would evaporate when operating. In such a xenon glow lamp, the positive column existing between the cold cathode electrodes is small in diameter due to the small amount of the lamp current. A desirable luminance distribution can not be achieved. This is because such a thin positive column fluctuates during the operation. The fluctuation of the positive column varies from time to time, and therefore, the luminance distribution is not stable.